Bombing Hitler's Supergun

Spring 1943. World War II is slowly beginning to turn in favor of the Allied forces. But, growing desperate, Hitler hatches plans for a diabolical weapon: a bank of “superguns" housed in a massive underground complex in Nazi-occupied northern France. Together the guns would be able to pump 300 heavy high explosive shells into downtown London every hour—a target 100 miles away. This weapon could spell doom for the Allies. But how can such a massive gun possibly work? Join NOVA as engineers, archeologists, and World War II historians investigate this fearsome weapon. And discover the two audacious missions designed to destroy the seemingly impregnable supergun complex, one of which is led by none other than Joseph Kennedy Jr. Can these hair-raising missions save the Allied forces?

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Bombing Hitler's Supergun

PBS Airdate: May 11, 2016

NARRATOR: It is the height of World War II; Allied intelligence officers spot something terrifying: the muzzle of an enormous cannon, protruding from an underground Nazi bunker. It is a supergun, a monstrous new weapon, part of Hitler's plan to reduce London to rubble and win the war through terror.

TONY POLLARD (Battlefield Archaeologist): It's a very cruel and a very nasty way of making war, but they believed it might work.

NARRATOR: The Allies hatch two bold plans to defeat it. One involves Joe Kennedy, Jr., eldest son of what would become an American political dynasty. He would be piloting an explosive drone.

NICK SPARK (World War II Drone Expert): What happened to Joe Kennedy and his co-pilot on that plane is actually one of the greatest mysteries of World War II.

NARRATOR: The other scheme would use the biggest bomb the world had ever seen. But would either plan work?

HUGH HUNT (Engineer): Three, two, one.

NARRATOR: Now, in a series of explosive experiments, engineer Hugh Hunt will investigate the allies' bunker-busting technologies.

NARRATOR: …to see if the weapon really could have brought London to its knees. Bombing Hitler's Supergun, right now on NOVA.

By spring of 1943, the tide of World War II was beginning to turn against Hitler. In an effort to regain the offensive, he drew up plans for the world's biggest gun. With 25 barrels, this enormous cannon would be buried deep underground, in Nazi-controlled France. It was designed to fire shells 100 miles, cripple London and pave the way for a Nazi victory.

In a desperate race to knock out Hitler's supergun, the allies devised their own miracle weapons. The Americans pioneered the drone, a radio-controlled heavy bomber, packed with 12 tons of high explosives. Leading the drone mission was 29-year-old Joe Kennedy, Jr., a man groomed from birth to be the first Catholic president of the United States.

The British drafted in the brilliant engineer Barnes Wallis. He came up with the original bunker-buster: a bomb that would explode underground and trigger a manmade earthquake.

The Allied efforts were focused on the tiny hamlet of Mimoyecques, five miles inland from the French coast. On the surface, it's now just a ruin, but hidden inside this hollowed out hill is the secret installation built to house the V-3 Nazi supergun.

TONY POLLARD: Good grief!

HUGH HUNT: I can't quite make sense of this. This isn't natural rock, this is concrete.

TONY POLLARD: No, this is a roof. Look at it. It's four or five meters thick.

NARRATOR: Engineer Hugh Hunt and battlefield archaeologist Tony Pollard have been drawn to find answers to the many questions that remain about this mysterious super weapon.

HUGH HUNT: This was the entrance of a tunnel?

TONY POLLARD: Yes, it looks as though there is some sort of hatchway.

NARRATOR: How did it work? Was it powerful enough to hit London, 100 miles away? And could it have ended the war? These are the mysteries that Tony and Hugh are trying to solve.

HUGH HUNT: This is a glorified pillbox, with a 130-meter-long gun. I'd love to see what's underground.

Right. Let's go.

NARRATOR: The fortified bunker has not been fully explored since the war.

TONY POLLARD: Whoa, that's slippy. That's slippy. You okay?

HUGH HUNT: Yep, yep!

NARRATOR: They rappel down one of the steep shafts that would have each contained five barrels of the supergun.

TONY POLLARD: Wow, look how steep it looks from the bottom! This is certainly big.

HUGH HUNT: This, we reckon, is about 50 meters.

TONY POLLARD: And it was 130 meters.

HUGH HUNT: And this is one third of the way down.

NARRATOR: The shaft extends a further 300 feet beneath them, but now it's blocked by rubble. Today, there is no sign of the barrels, but the design of the shaft reveals a telling detail.

HUGH HUNT: This tunnel is centered, very accurately, on a particular line. Well, here we are here at Mimoyecques, and this particular tunnel, that we see here, points directly over here at Westminster Bridge, which is astonishing!

NARRATOR: Hitler's gun was trained right on central London.

By the spring of 1943, the Nazis had tasted defeat in the deserts of North Africa, and they had been driven from the Soviet Union after a bloody battle at Stalingrad. Hitler was determined to strike back.

Taking center stage in his war room was the supergun, or the “London Cannon,” as he called it.

Its five shafts would each contain five barrels. That's a total of 25 barrels, firing 300 shells an hour, 24 hours a day. Maintaining this onslaught would require an enormous infrastructure: a network of galleries to store ammunition; 1,200 troops to man the guns and an underground railway to supply them.

This was artillery warfare on an industrial scale.

HUGH HUNT: Whoa!

NARRATOR: Hugh and Tony explore the farthest reaches of the complex, looking for clues to how the tunnels were built.

NARRATOR: What they find shows them the tunnels were carved by hand, using pickaxes and jackhammers with steel spikes.

TONY POLLARD: Oh, look! Look! Look at that, look at that!

HUGH HUNT: That fits.

TONY POLLARD: They're just using them to prise away fragments of the stone. And imagine the effort, all day every day, hammering these spikes in and then moving away all the rubble. It's just a horrible thought.

NARRATOR: The supergun's first victims were not the people of London, but the slave laborers who built the installation.

TONY POLLARD: This complex was built by a large number of people that had been conscripted against their will. And there can be no doubt that many, many people lost their lives. This wasn't a normal building site; work here never stopped. They had gangs of workers on site, 24 hours a day, 7 days a week. The priority here was to get those guns into action, and the human cost didn't matter.

NARRATOR: Hitler wanted to rush his wonder weapon into action, because he was hungry for revenge. He had bombed London during the Blitz, in the early days of the war, but, since then, his Luftwaffe had lost air superiority in the skies over Europe. The allies were exploiting this lack of air cover by relentlessly bombing German cities.

Hitler began building fortified installations all over Northern France: massive concrete bunkers, hidden in remote woods, that would house a new generation of secret armaments, called “V-weapons.”

TONY POLLARD: They're called the V-weapons because they're about vengeance. They're about retribution. This is Hitler getting his own back. What the R.A.F. and the American Air Force have been doing is bombing German cities and killing German civilians. And Hitler is absolutely outraged by this, and decides he's going to take it out on London. So, all of these weapons are designed to hit London from France.

NARRATOR: These were technologies straight out of science fiction. There was the V-1, a jet-powered flying bomb. There was the V-2, a rocket that would shoot beyond the stratosphere before falling back to Earth. These weapons had the potential to reduce London to rubble, but they were untested in battle and unreliable.

TONY POLLARD: Initially, at least, Hitler was fairly skeptical about these new experimental weapons. Rockets and missiles, not really his bag. His was a more traditional military background, molded in the trenches of the First World War, but what that experience did give him was knowledge of how effective artillery could be. He'd seen how artillery could cut men apart. Upwards of 80 percent of the casualties on the Western front were caused by artillery.

And the V-3 was a supergun. It was old school, but with a new twist.

NARRATOR: The twist was that, unlike traditional artillery, the shells of the V-3 supergun would not be falling on frontline soldiers, but raining down on London's terrified civilians.

In September 1943, in a country house, in Buckinghamshire, the first evidence began to emerge of Hitler's secret plot against the British capital. An elite team of specially trained intelligence officers had been posted here to comb through aerial photographs, taken on reconnaissance missions over occupied northern France. They were searching for signs of Nazi weapons sites.

Officers examining pictures of the Mimoyecques area spotted something mysterious: railway tracks disappearing into a hill. They couldn't see what was going on underground, but they wondered if the complex concealed a new kind of weapon.

For two years, a corner of Britain had already been on the receiving end of big Nazi guns. And the K5 railway gun, just outside Calais, in France, was one of the biggest.

TONY POLLARD: It was used for pounding the English coast, and, in fact, they would quite regularly hit the town of Dover. And that corner of southeast England got pounded so heavily through the years of the war it became known as “Hellfire Corner.”

What the population would do would be to run into the caves, in the cliff, behind the town. And the Germans were a bit cruel in that they would wait until the “all clear” had been sounded, until people started to come out again, and they'd fire another one to try and catch them in the open. They hit the coast very hard. Something like 10,000 houses in Kent had been destroyed by this artillery fire.

NARRATOR: The K5 could fire a shell 40 miles, but Hitler wanted to reach London, more than double the distance. To get there, his engineers had to find a way of increasing the speed of the shell. One method was simply to increase the length of the barrel. This kept the bubble of rapidly expanding gas from the gunpowder blast pushing on the shell for longer.

But lengthening the barrel causes a problem.

HUGH HUNT: One thing that people don't necessarily think about is that this bends under its own weight.

TONY POLLARD: Really? Even a gun barrel?

HUGH HUNT: Even a gun barrel. And the longer you make it, the more flexible it's going to be.

NARRATOR: But supporting a longer barrel isn't the only problem. If the barrel is extended too far, the expanding gas propelling the shell will eventually peter out.

One possible solution was to increase the amount of gunpowder, but that would risk blowing the barrel apart. A German military engineer called August Coenders came up with a radical proposal. He would not set off the whole charge in one go. Instead, he would split it into smaller amounts and place them in chambers along the length of the gun. After the shell was fired conventionally, the cascade of explosions would boost the speed of the shell as it travelled down the barrel.

Hitler ordered his engineers to build a prototype supergun in German-occupied Poland and stepped up work on the stronghold that would house it, in France.

Then, in the fall of 1943, the Allies took the first steps to knock out the site. Two-hundred-and-nineteen bombers took off from airfields around Britain. The American and British bombing campaign to destroy the mysterious building site at Mimoyecques had begun.

In total, they would fly 18 missions; 1,375 aircraft would drop 6,517 bombs. But conventional bombs made little impact on the fortifications buried deep underground. The allies were forced back to the drawing board. Engineers working on both sides of the Atlantic tried to find a powerful enough weapon to destroy sites like this in a single blow.

The British called on their go-to engineering genius, Barnes Wallis. Wallis was an original thinker, with ideas years ahead of their time. He had designed one of the most successful aircraft of World War II, the Wellington Bomber.

But it was the weapon he invented to destroy the hydroelectric dams that powered the Nazi war machine that made his name: the legendary “bouncing bomb.” The success of the dam-busters raid, carried out in May 1943, by the Royal Air Force 617 Squadron, secured Wallis' position as Britain's top military engineer.

Now, his challenge would be to create the biggest bomb the world had ever seen and use it to trigger an earthquake that would shake a powerful structure apart.

The American military began experimenting with an extraordinary innovation of its own, the drone. The idea was to pack an aircraft with explosives and turn it into a flying bomb. A pilot would take the plane into the air, before bailing out, and then a remote pilot would guide the aircraft to the target by radio control.

Volunteering for this most hazardous mission was a 29-year-old naval aviator. He was the eldest son in a family that would become a great political dynasty. His name was Joe Kennedy, Jr., son of Joseph Kennedy, one of America's richest men and former ambassador to the U.K. Joe's path through life had already been mapped out by his family.

CARI BEAUCHAMP (Kennedy Biographer): His grandfather, upon his birth, had announced to the press that his grandson would be the first Catholic president of the United States. So, from day one, literally and figuratively, Joe, Jr. had this focus on him, that that was his destiny. I mean, it was very much like an heir to the throne the way he was raised.

NARRATOR: Joe was a naval pilot patrolling the Atlantic Ocean, on the lookout for German U-boats. With more than 50 missions to his name, he never caught so much as a glimpse of an enemy submarine. Still, he could have returned home with honor, but the chance to lead the attack on Hitler's supergun was irresistible.

CARI BEAUCHAMP: I think he really believed that this would be his contribution. He didn't think that what he had done so far was enough. Yet, to everyone else, it was more than enough. But he felt compelled to keep performing.

NARRATOR: Neither Kennedy nor his team fully appreciated the threat that London was now under. By May 1944, the Germans were on the verge of reaching the British capital with their prototype supergun.

They had managed to fire shells 80 miles, at their secret test site in Poland, further than any other shell in history. But just how gun designer Coenders managed to time the booster charges to go off one after the other, with microsecond precision, remains a mystery to this day.

One theory is that he used the hot gasses behind the shell to ignite the boosters, as the shell passed down the barrel. Engineer Hugh Hunt wants to test that theory, so he has asked explosives engineer Charlie Adcock to build him a supergun of his own. They will use the barrels of six rifles, fit each section with a booster chamber, and join them end-to-end.

If Hugh can get his experimental model to function, it will help him to solve the mystery of how Hitler's supergun might have worked.

HUGH HUNT: There's a couple of things we want to look at here. One is we want to see if it was indeed the hot gasses that initiate the booster charges. And the other thing is do the booster charges make the bullet any faster?

NARRATOR: They will shoot the bullet through two beams of infrared light. The time it takes for it to pass between them will give an accurate speed.

CHARLIE ADCOCK (Explosives Engineer): Live round.

NARRATOR: First, they will fire the gun without the boosters to set a benchmark speed.

CHARLIE ADCOCK: We are all good. We can retire to the chamber.

Quick blast on the hooter. Okay. Firing.

HUGH HUNT: There we go. That's got a velocity, 742 meters per second.

NARRATOR: Now, they will try to speed up the bullet with the booster charges.

HUGH HUNT: Let's load up these side chambers. So, these booster charges, what have you put in them?

CHARLIE ADCOCK: These are rifle cartridges, full of high explosives.

HUGH HUNT: Charlie's got a fantastic instinct for what's going to work. And handling these high explosives is a bit scary, but what I'm most interested in is whether it's going to make this projectile go faster.

CHARLIE ADCOCK: Stand by. Firing!

HUGH HUNT: Whoa, 964; that's nearly a thousand meters per second.

NARRATOR: Hugh has made a key observation. He can clearly see that the boosters were set off, as expected, by the hot gas behind the shell.

HUGH HUNT: What have we got?

CHARLIE ADCOCK: Everything's fired. Yeah, they've all gone.

NARRATOR: And, as Hugh discovers, there's an added bonus.

HUGH HUNT: There's been a 30 percent increase in speed, and that's going to translate to maybe a 60 percent increase in range. In terms of what Hitler might have been wanting to achieve with his gun, booster charges massively increased the range.

NARRATOR: And now that Hitler's supergun was nearing completion, there was a new urgency for him to use it.

On 6th June, 1944, D-Day, 150,000 Allied troops stormed the beaches of Normandy, in the biggest invasion in history. In just 24 hours, they smashed through the German defenses and gained a toehold in Nazi-occupied Europe.

Hitler demanded a rapid response. London was the target. Within a week of D-Day, he had pressed the first of his Vengeance weapons into action. The jet engines of the V-1 flying bombs were timed to cut out over the city. Up to 100 bombs fell from the sky every day. Over an 80-day period, about 6,000 people were killed and a million buildings damaged or destroyed.

TONY POLLARD: The philosophy was that if you could destroy the morale of a people, you would, basically, break their will to fight. And so, rather than go for the hard military targets, you would go for the soft civilian targets. It's a very cruel and a very nasty way of making war, but they believed it might work.

NARRATOR: In July 1944, at a secluded American airbase in the east of England, a specially modified bomber, loaded with top-secret equipment, flew in from Philadelphia. This was the centerpiece of the daring American plan to wipe out Hitler's supergun. Instead of merely dropping its payload on the target from above, the aircraft itself would be used as a remote controlled bomb.

As leader of the mission, Joe Kennedy chose to downplay the jeopardy he faced.

CARI BEAUCHAMP: He writes his father, “I'm going to stay just for one more mission. There's hardly any danger. I'm sure it'll be fine.” And Joe, Sr., while incredibly disappointed, wrote back and says, “Please, I understand, just don't push your luck.”

I think he had a sense of a little bit of a golden aura about him, and I think he really believed he could pull it off.

NARRATOR: Kennedy's aircraft would be loaded with 12 tons of torpex high explosives, then he and his co-pilot would take the plane up to 2,000 feet, accompanied by another aircraft, known as the “mother ship.” Using a state of the art guidance system, the mother ship would take control of the aircraft, while the crew parachuted out, itself a risky maneuver.

NICK SPARK: Bailing out of one of these drones was very dangerous, because the plane was flying at very high speed. You might be jumping out into a 200-mile-an-hour slipstream. There were several deaths that occurred, and at least one amputation where someone's arm got caught up in the parachute shrouds.

NARRATOR: Once the crew had bailed out, an operator in the mother ship sent a radio signal to prime the explosives for detonation. They would then explode on impact. This was the world's first precision-guided-attack drone, made possible by a piece of remarkable new technology.

NICK SPARK: What's really amazing, and I think it's something that most people don't appreciate, is that before television arrived in your house, it had actually already been used on the battlefield.

What we have here is an actual World-War-II-era Iconoscope television camera. And it's absolutely authentic. It hasn't even been restored. And we've got it hooked up to a T.V. monitor, and I've pointed it a somewhat distant object, American flag up on the pole, there.

Now, I can make out the stripes, no problem, but I really can't see the stars very well at all. But, this was good enough to use for a military application. Adding television into the system meant that they could see exactly what the weapons saw, as it tracked into the target. And that was a revolutionary development.

NARRATOR: The plane was fitted with two prototype television cameras. One pointed straight ahead, showing the way to the target. The other was focused on the gauges on the dashboard. The live pictures were relayed back to the operator in the mother ship, who used a joystick to manipulate the controls of the unmanned aircraft.

NICK SPARK: The T.V. camera made the drone a truly precise weapon, something you could fly by remote control, see where it was headed. And you could, literally, put yourself in the cockpit of that aircraft. It was going to be able to deliver that 12 tons of torpex directly on this target. It was not going to miss.

NARRATOR: Neither Kennedy's team, nor their British counterparts, had much idea of the scale of the task they faced taking on the supergun.

Once installed, the barrels would be encased in rock over 300-feet thick, and shielded by 16 feet of concrete. And, so far, the complex had been impregnable to Allied bombs.

But, fortunately for the British, they could call on the mastermind behind the bouncing bomb. Barnes Wallis had already proven his worth by busting the formidable German dams, but could he crack a Nazi stronghold like Mimoyecques?

IAIN MURRAY (Barnes Wallis Biographer): The thing which runs through all of his work is efficiency, he was always after the most efficient solution to the problem. What he realized was that it was more efficient to destroy a factory than it is to destroy all of the tanks or airplanes that come out of the factory. And it's even more efficient, if you can destroy the power sources that the factories were using.

NARRATOR: Wallis started thinking about the most efficient use of explosives and came up with a brilliantly original idea: a bomb that would explode underground and create an earthquake.

Now, Hugh Hunt wants to see if a bomb going off underground would be more destructive than one exploding on the surface.

HUGH HUNT: Well, suppose that this is a bunker in Northern France, and you want to blow it up. Barnes Wallis' method was to bury a bomb deep in the ground, to create a mini-earthquake. You only have to push the foundations a short distance, and you've destroyed the structure. We're going to do two tests. One test is with some explosive on the ground nearby, and then we'll bury that same amount of explosive deep in the sand, and we'll see what happens then.

Firing, three, two, one!

The pressure has created a crack in the building. I think it will be interesting to see, with the same amount of explosive, what happens when we put it in the ground underneath the building.

IAIN MURRAY: I don't think anybody had looked at the effect of bombs exploding underground, at that point. People instinctively thought it was the bit of the bomb exploding that you could see visibly, which did all the damage. But, in fact, that's only part of the damage. Most of the damage is caused by the pressure wave, which is actually invisible.

NARRATOR: With the help of slow motion photography, it is possible to see the pressure wave moving, at supersonic speed, ahead of the blast, rippling the ground, as it travels along.

Wallis had found that, above ground, this wave of energy would quickly dissipate and do comparatively little damage, but below the ground, through solid earth, it would travel with more destructive force.

So, pound for pound, an underground bomb would be more efficient than one exploding on the surface, but getting it to penetrate deep into the earth was the challenge.

Each bomb was laboriously sculpted, on a lathe, into the perfect shape, that would allow it to pierce the ground and move the earth aside as it burrowed in. The steel casing had to be just thick enough, where strength was most needed, but would taper away, elsewhere, to save weight.

IAIN MURRAY: So, this section, down here, is virtually solid, for hitting the ground very fast, without squashing up. That's really solid. And then, that's nice and hollow, down that part. It's only about an inch thick, at this point. And then, this would have penetrated into the ground to 50 feet or more. And this would all have exploded. And the massive shockwave would have permeated out, through the ground, and anything nearby would have been shaken to pieces.

NARRATOR: The R.A.F. started ordering up earthquake bombs. It was not a moment too soon.

In a dramatic breakthrough, intelligence officers studying photographs of Mimoyecques spotted suspicious openings in a concrete slab that protected the supergun's underground bunker. Just visible, in one of the openings, was an object they identified as the end of a barrel. It looked like the Nazis had started installing the supergun.

The response from the British was immediate. The elite 617 Squadron was called in. Famous for its dam-busting raid with Wallis' bouncing bombs, the squadron was chosen to deliver his latest weapon to Hitler.

JOHN BELL (617 Squadron): You were with a group of really experienced crew, people who'd been on the dams raid. You looked up to them. And here was a squadron equipped with yet another Barnes Wallis weapon. It had to be good. It was that sort of feeling in the squadron. You really got something with which you could hit the enemy with. And here was a beautifully streamlined bomb, and it really looked as though it was going to do the business, like throwing a dart at the board and getting a bullseye every time.

NARRATOR: And this would be a precision strike. Seventeen bombers, each equipped with a single Barnes Wallis bomb would attack in broad daylight.

This rare footage shows the earthquake bombs exploding.

Interpreters examining photographs of the hill, taken immediately after the raid, identified eight enormous craters made by Wallis' earthquake bombs. Although the concrete slab had been hit, it was impossible to tell just how much damage the bombs had done below ground, so the hill remained a prime target.

One month later, it was the Americans' turn to attack the supergun site. Joe Kennedy was ready to lead the experimental mission to turn a heavy bomber into a remote-controlled missile.

But, as the plane sat on the apron, a young officer named Earl Olsen spotted what he thought was a flaw in the system. On the control panel, designed to arm the explosives, a special safety pin had been inserted. This was supposed to prevent a rogue radio signal, one that hadn't been transmitted by the mother ship, from accidentally arming the bomb.

Earl was worried that jamming the arming panel with the pin might damage another part of the system. He took his concerns right up the chain of command, but no one would listen.

NICK SPARK: Out of everybody in the field with the drone program, Earl Olsen was the most qualified person to make an assessment of this panel. But he was dismissed by a number of people, because he was a junior grade officer. He was not a college graduate. A lot of what he knew about electronics was self-taught, and the people at the Naval Aircraft Factory who had built this panel were viewed as experts, as world class experts, and who was Earl Olsen to argue with them?

NARRATOR: Exactly what the flaw was that Earl Olsen spotted has been lost. Finding the one document that might identify it, the circuit diagram of the panel, has long obsessed Nick Spark.

NICK SPARK: I just kept looking. I looked in every nook and cranny there is: the National Archives; contacted the Navy and filed Freedom Of Information Act requests; and finally, after about two years, in a pile of documents that I had received, I turned over a page, and there it was: a diagram of this arming panel. So, when I saw that, I knew I had found the smoking gun.

NARRATOR: Using Nick's diagram, Hugh Hunt reconstructs the panel to try to uncover the fault Olsen had identified.

HUGH HUNT: When a radio signal comes in from the mother ship to arm the explosives, it essentially powers up this board, like this. They'd, perhaps rather sensibly, put in a safety pin, so that the mechanical arming system would never accidentally operate. And if there was a rogue radio signal, like this, then the lever doesn't move, which means that the mechanical arming system is safe. But look, this light is still on, and that means the electrical power continues to run to these little solenoids that are not designed to have current flowing into them continuously.

NARRATOR: The jammed lever has inadvertently held down a switch that allows electricity to flow, illuminating the bulb and overheating the solenoid. This could start a fire and set off the explosives.

These tiny components, called solenoids, were part of the aircraft's back-up arming system. They would only be activated if the unmanned drone ever went out of control over Britain, and it was necessary to blow it up in midair. In such an emergency, the mother ship would signal the arming panel to send a pulse of electricity to the solenoids. These would then pull out pins from the detonators, priming the explosives to go off. The plane could then be safely blown up in midair.

But the problem was, if the arming panel received a rogue radio signal, as Olsen feared, the jamming action of the safety pin would allow electricity to flow continuously to the solenoids, and this would cause them to dangerously overheat.

This explains why Olsen was so desperate to delay the mission.

Before his death, in 2011, he described his final conversation with Joe Kennedy.

EARL OLSEN (VIDEO CLIP): He said, “Well, Earl, is there any sure thing that will cause me trouble?” And I said, “Well, I can't tell you. It may cause you trouble right away, or it may be delayed a while. It takes a little while, probably, for that thing to heat up enough to set off the charge. After 15 minutes or more, it might blow up, then. If you please, take off and drop out, as soon as possible, you might escape it.”

NARRATOR: Zero hour had arrived for the drone strike on Mimoyecques. Joe Kennedy decided he would take his chances. At six p.m., he and his co-pilot squeezed past the 12 tons of high explosives and the suspect arming panel.

All Earl Olsen could do was watch and pray. An armada of support aircraft followed Kennedy's plane into the air. Olsen was able to watch the events unfold, as television pictures from the drone were transmitted back to the airfield.

NICK SPARK: Kennedy's plane, “Zoot Suit Black.” had a perfect takeoff. It climbed to about 2,000 feet, which was the mission altitude, and, at that point, Kennedy went on the radio and said the code word, “Spade Flush,” twice. That was the signal for the control pilot in the mother ship to take over command of the airplane, and he began to test it methodically: turning the television on and off and putting the plane into various turns.

NARRATOR: No one will ever know for sure, but it seems likely that some time after takeoff, a stray radio signal was picked up by the arming panel, triggering the chain of events that Earl Olsen had feared.

The aircraft exploded over England, almost 100 miles from the target. No human remains were ever found.

Joe Kennedy, Sr., received the news the following day.

CARI BEAUCHAMP: He was devastated beyond words. All his plans, everything that had happened to him, he could understand and he could cope with. But having this happen to his son was beyond devastating.

NARRATOR: To Olsen's deep regret, his prediction had come true.

NICK SPARK: He had tried so desperately to prevent them from flying, and even at the risk of court martial. And he was not a drinker at all, but he got dead drunk the night that that happened. And it stayed with him throughout his life.

NARRATOR: Joe Kennedy and his co-pilot Bud Willy gave their lives to save the people of London. But just 23 days after their fateful flight, allied soldiers made a discovery that cast the two Americans' sacrifice in a tragic light.

On the 5th of September, 1944, Canadian troops fought their way through to Mimoyecques and found the bunker in ruins. The British attack the previous month had destroyed Hitler's supergun, after all.

The earthquake bomb created by Barnes Wallis had lived up to its name.

HUGH HUNT: All of these carefully constructed arches of concrete, and all the foundations for the gun itself, and for the infrastructure for the gun was damaged way, way beyond repair. Many eyewitnesses at the time described it as being like an earthquake.

NARRATOR: In the nick of time, Hitler's plot to destroy London had been foiled, and with it, his hopes of turning the tide of war in his favor lay in ruins.

The shockwaves from the American supergun raid reverberated way beyond the borders of Europe, especially for Joe's younger brother, Jack Kennedy.

CARI BEAUCHAMP: Jack gets the word of his brother's death, this incredible golden brother, who he's adored and revered since birth, and yet it also means something else to him, and he's very well aware of it. And as he says to his friend, “Now the burden falls to me.”

NARRATOR: Jack Kennedy picked up where his golden brother had left off, and in 1961 was inaugurated as the first Catholic President of the United States.

Two years later, at a naval test site in California, the president came close to meeting the man who tried to save his brother's life.

NICK SPARK: When John F. Kennedy came and visited Point Mugu, Earl Olsen worked there. And people there knew that he had worked with Joe Kennedy in the war, and they said, “Hey, don't you want to meet John F. Kennedy?” And he said, “I can't do that. I wouldn't want to have to tell him the truth about what happened to his brother.”

NARRATOR: Seventy years after the war, bunker-busting bombs are still part of military arsenals, and drones are in common use, but no nation has ever successfully deployed a supergun.